1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus and a control method thereof, and more particularly, to an ultrasonic diagnostic apparatus configured to three-dimensionally scan the inside of a body under examination using an ultrasonic wave in response to a trigger signal generated based on an electrocardiogram signal or the like, and a method of controlling such an ultrasonic diagnostic apparatus.
2. Description of the Related Art
In recent years, an ultrasonic diagnostic apparatus capable of displaying a three-dimensional moving image has been in active development, and it has become possible to display a three-dimensional diagnostic image with higher resolution over a larger region than in conventional two-dimensional images.
The ultrasonic diagnostic apparatus generates a diagnosis image using an ultrasonic wave propagating in a living body, and thus the time from the transmission of an ultrasonic pulse to the reception of a reflected wave from a living body is basically the same for a three-dimensional ultrasonic diagnostic apparatus and a two-dimensional ultrasonic diagnostic apparatus. To scan a three-dimensional region in a living body with high resolution, a great number of scanning beam positions are required. Thus, the three-dimensional ultrasonic diagnostic apparatus generally needs a longer time to scan a specified region than the two-dimensional ultrasonic diagnostic apparatus needs. In other words, when the spatial resolution is equal, the frame rate of the three-dimensional image (i.e., the frequency at which the three-dimensional image is updated) obtained by the three-dimensional ultrasonic diagnostic apparatus is theoretically lower than the frame rate of the two-dimensional image obtained by the two-dimensional ultrasonic diagnostic apparatus.
To solve the problem described above, various techniques have been proposed (see, for example, U.S. Pat. No. 6,544,175, JP-A 2007-20908, etc.). A basic idea of these techniques is to divide a full region (volume) under examination for diagnosis (hereinafter, referred to simply as a full volume) into a plurality of small regions (hereinafter referred to as sub volumes), and obtain a three-dimensional image of the full volume by connecting image data obtained by scanning three-dimensional space of the sub volumes at a high frame rate. In this technique, the observation time of sub volumes varies from one to another. Therefore, it is important to connect sub volumes so that good spatial continuity is achieved.
Depending on a part under diagnosis, the part can move due to breathing or a heartbeat. To avoid a problem due to the motion of the part under diagnosis, for example, U.S. Pat. No. 6,544,175 discloses a technique to acquire a plurality of image data in a sub volume in synchronization with the motion of a heart. In this technique disclosed in U.S. Pat. No. 6,544,175, a three-dimensional moving image of a heart is produced in real time as described briefly below.
In this technique, a signal of an electrocardiogram, i.e., an ECG signal is used as a signal synchronous with motion of a heart. More specifically, an R-wave signal, which appears at the end of a diastolic period, is used as an ECG trigger signal.
A three-dimensional full volume of a heart under examination is divided into, for example, four sub volumes, and image data of one heartbeat is captured in synchronization with the ECG trigger signal for each sub volume. Note that the image data of one heartbeat includes a plurality of frames of images. For example, 20 frames of images of one sub volume are obtained by repeatedly scanning the sub volume 20 times for one heartbeat (during one interval of the ECG trigger signal). In this case, if the repetition period of the heartbeat is assumed to be one second, the image data of each sub volume is obtained at a frame rate of 20 fps, which is reasonably high to obtain a moving image representing motion of a heart.
The plurality of frames of image data obtained for each sub volume are connected to obtain a full volume of image data as follows. That is, frame images that are same in “time phase” are extracted from the plurality of fame images of sub volumes and are connected together so as to obtain a frame image of the full volume. The “time phase” refers to a delay with respect to a time at which an ECG trigger signal is generated. The motion associated with contraction and relaxation of the heart normally has periodicity synchronous with the ECG trigger signal. Therefore, by extracting frame images which are equal in the time phase from the respective sub volumes and connecting the extracted frame images, it is possible to obtain good spatial continuity between the sub volumes. In practice, successive “time phase numbers” are assigned to frame images in scanning order from one closest to an ECG trigger signal, and an image of a full volume is synthesized by connecting frame images having an equal time phase number. For example, in a case where the full volume is divided into four sub volumes A, B, C, and D and each sub volume is scanned repeatedly 20 times, a total of twenty frame images with time phase numbers of 0 to 19 are obtained for each sub volume. Frame images with each equal time phase number are extracted from the sub volumes A, B, C, and D and the extracted frame images are connected together thereby obtaining a synthesized image of the full volume corresponding to the time phase number. The combining of frame images is performed for each of the time phase numbers so as to obtain synthesized full volume images with time phase numbers from 0 to 19. Thus, a total of twenty synthesized full volume frame images are obtained for each ECG trigger signal. Note that the frame rate of the full volume images is equal to that of the sub volume images. Thus, for example, a full volume moving image with a frame rate of 20 fps is obtained.
As described above, in the conventional technique disclosed in U.S. Pat. No. 6,544,175, each sub volume is scanned a plurality of times in response to each ECG trigger signal. In the repeatedly performed scanning operation described above, in general, the sub volumes are scanned while changing the transmission direction of the ultrasonic beam from one transmission pulse to another. Thus, in general, the time needed for each scanning (hereafter, referred to as a scan repetition period (the reciprocal thereof corresponds to the frame rate of the sub volumes)) is determined by the product of the pulse repetition period of the transmission pulse and the number of beam positions of the transmission ultrasonic beam.
Of these parameters, the pulse repetition period is limited by the maximum diagnosis distance of a part to be examined (the depth of the part to be examined). If the pulse repetition period is too short, the maximum detectable depth becomes small. Conversely, if the pulse repetition period is too long, the scan repetition period of each sub volume becomes long and the frame rate decreases, which results in a reduction in time resolution of the moving image.
In view of the above, in general, the pulse repetition period is set to a constant value that allows the frame rate to become as high as possible within a range that allows the required maximum diagnostic distance to be achieved, and the transmission beam position is changed every pulse repetition period set to the constant value.
On the other hand, the number of transmission beam positions in the sub volume is determined by the area size of the sub volume, i.e., the scanning range of the sub volume. When the number of sub volumes into which the full volume is divided is constant, the area size of one sub volume is determined by the area size of the full volume and thus the number of transmission beam positions is determined by the area size of the full volume, which is nearly equal to the area size of the part to be examined for diagnosis.
The pulse repetition period and the number of transmission beam positions are determined by the depth and the area size of the part to be examined for diagnosis in the above-described manner, and thus the repletion scanning period is determined. Therefore, when the depth and the area size of the part to be examined are fixed, the scan repetition period is allowed to be set to a constant value. Thus, in the conventional technique, the scan repetition period is set to a constant value.
Incidentally, as described above, the repetitive scanning operation of one sub volume is started in response to an ECG trigger signal, and the same sub volume is scanned repeatedly until a next ECG trigger signal comes. If the next ECG trigger signal comes, the repetitive scanning operation on a next adjacent sub volume is started, and this sub volume is scanned repeatedly.
Therefore, if the interval of the ECG trigger signal, i.e., the heartbeat period is not equal to an integral multiple of the predetermined constant scan repetition period, the scanning operation in a period immediately before the arrival of an ECG trigger signal is aborted before the scanning in that period is completed. As a result, the image acquired in this repetition period is incomplete and useless, and thus the obtained image is discarded without being used. This causes a reduction in the efficiency of using the acquired data. Besides, it becomes impossible to obtain a heart image in the state immediately before the ECG trigger signal.
The above mentioned problems arise not only for the case where the full volume is divided into the sub volumes but also for the case where the full volume is scanned without division into the sub volumes, so long as the ECG trigger signal is used for a starting signal of the scanning of the sub volume or the full volume.
Thus it is desirable to provide an improved technique to solve the above problem in diagnosis.